1. Field of the Invention
The present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs), and more particularly relates to a lamp inverter with a pre-regulator for improved efficient operation of the CCFLs.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate the light is limited. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFLs are used for back lighting or edge lighting of liquid crystal displays (LCDs), which are typically used in notebook computers, web browsers, automotive and industrial instrumentation, and entertainment systems.
CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube, which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphorescent material coated in the inner wall of the tube, resulting in visible light.
A power conversion circuit is generally used for driving the CCFL. The power conversion circuit accepts a direct current (DC) input voltage and provides an alternating current (AC) output voltage to the CCFL. The brightness (or the light intensity) of the CCFL is controlled by controlling the current (i.e., the lamp current) through the CCFL. For example, the brightness of the CCFL is related to an average power provided to the CCFL. Thus, the brightness of the CCFL can be controlled by changing the amplitude of the lamp current (e.g., amplitude modulation) or by changing the duty cycle of the lamp current (e.g., pulse width modulation).
One type of power conversion circuit uses pulse width modulation techniques to drive the CCFL. The power conversion circuit varies the pulse widths (or the duty cycles) of one or more driving signals to control the average power provided to the CCFL. The pulse widths of the driving signals may be varied to compensate for variations in the input voltage or to achieve a desired brightness.
One problem with varying the pulse widths of the driving signals is that lamp efficiency may degrade. Lamp efficiency in terms of light output versus lamp current amplitude degrades with increasing lamp current crest factor. Lamp current crest factor is defined as a ratio of the peak lamp current level to the root mean square (RMS) lamp current level. Lamp current crest factor increases as the pulse widths of the driving signals are reduced from an optimum level. For example, lamp efficiency generally degrades as the pulse widths of the driving signals are reduced to dim the CCFL or to compensate for an increased in the input voltage.
Driving signals with shorter pulse widths decrease the duration that power is coupled to a secondary winding of a transformer in the power conversion circuit, resulting in less power provided to the CCFL for a given supply voltage. At the same time, the driving signals with shorter pulse widths decrease the effectiveness of signal filtering provided by the transformer's leakage inductance and output capacitance, resulting in a greater difference between the peak lamp current level and the RMS lamp current level. Thus, the driving signals with shorter pulse widths result in higher lamp current crest factors and lower lamp lighting efficiency.